Conventionally, a method of irradiating an object with radiation and detecting the intensity distribution of the radiation that passes through the object in order to obtain a radiographic image of the object is widely used in industrial non-destructive testing, medical diagnoses and other fields. As a specific example of a typical method of obtaining a radiographic image of an object, there is one that employs a combination of so-called “phosphor sheets” (or intensifying paper), which emits fluorescent light when exposed to radiation, and silver-halide film, in which radiation passing through the object is converted into visible light by the phosphor sheet and the visible light forms a latent image on the silver-halide film, after which the silver-halide film is chemically processed in order to develop the latent image into a visible image. The radiographic image obtained by such a method is an analog image, used in diagnostics and testing.
By contrast, a technique for obtaining digital images using as the image receptor a two-dimensional array sensor comprised of picture elements, or pixels, arranged in a lattice or grid, with the pixels in turn comprised of tiny photoelectric converter elements, switching elements and the like, has recently been developed. A radiographic apparatus using this type of technology can display the acquired image data instantaneously, and is called a direct X-ray digital imaging apparatus. This type of digital radiographic apparatus has several advantages over the analog photograph technique, including the fact that it is filmless, that the images can be processed in ways that make the acquired information more useful, and that digital images easily lend themselves to the creation of databases
FIG. 5 is a schematic diagram of a radiographic system that uses the two-dimensional array sensor described above.
As shown in the diagram, X-rays emitted from an X-ray source 12 of an X-ray generator irradiate an object (in this case a person P), with the X-rays passing through the object P reaching a two-dimensional array sensor 14 inside a radiographic apparatus housing 100 placed between the object P and a table 13. The two-dimensional array sensor 14 has a phosphor sheet that renders the X-ray image as visible light. The X-ray image rendered as visible light by the phosphor sheet is then converted into electrical signals by photoelectric converter elements that are sensitive to visible light and are arranged in a grid. The image information resulting from the conversion into electrical signals is then digitized by an AD (analog-digital) converter, not shown, and the digitized image information is processed by an image processor 15 into digital image data. An image based on this image data is then displayed on a monitor 16. Moreover, the digital image data can also be stored in a pre-existing digital storage device 17.
FIG. 6 is a diagram showing a schematic cross-sectional view of the internal arrangement of the X-ray imaging apparatus described above.
Inside the housing 100 of the X-ray imaging apparatus are disposed a phosphor sheet 8 that renders the X-rays as visible light, photoelectric converter elements 18 arranged in a grid for converting the visible light into electrical signals, a glass plate 19 that supports the photoelectric converter elements 18 from the back (as seen from the side from which the X-rays come), a base 7 that supports the glass plate 19 and an electrical circuit board 1 that receives the electrical signals from the photoelectric converter elements 18 through a flat cable 20 and performs AD conversion of the photoelectrically converted signals. In addition, an X-ray shield member 21 is disposed between the glass plate 19 and the base 7. Further, elements 3 and 22, comprising an amp for amplifying the electrical signals from the photoelectric converter elements 18, an IC for controlling the driving of the photoelectric converter elements 18, etc., as well as a protective layer 5, are disposed on the electrical circuit board 1.
In a radiographic apparatus like that described above, the X-rays pass through the glass plate 19 to reach the constituent elements of the lower layer without being completely absorbed by the phosphor sheet 8. The X-rays also pass through these elements as well, but some of the X-rays are reflected back to the phosphor sheet 8 as secondary X-rays (also called scattered rays). When these scattered rays are rendered as visible light by the phosphor sheet 8 they cause deterioration in contrast of the X-ray image of the object. The X-ray shield member 21 is provided in order to shield against these sorts of scattered rays, for which lead (Pb), with its low rate of X-ray transmittivity, is widely used.
The probability of scattering occurring and the probability of the X-rays passing through depend on the structure of the matter, as well as on the nature of the quality of the used X-rays.
FIG. 7 is a schematic diagram expressing the state of passage of the X-rays through each of the constituent elements of the apparatus. For simplicity, the quality of the X-rays is assumed to be monoenergetic.
If the amount of X-ray radiation transiting the material without being absorbed by the phosphor sheet 8 and the photoelectric converter elements 18 is S and the probability of scattering at the glass plate 19 is Gs, then the amount of scattered rays X1 per unit of surface area of the glass plate 19 can be expressed asX1=S·Gs  (1)
Similarly, if the probability of scattering at the X-ray shield member 21, the base 7, the electrical circuit board 1, the element 3 and the protective layer 5 is Ps, Us, As, Bs and Cs, respectively, then the amount of X-ray radiation scattering X2, X3, Xa, Xb and Xc at the X-ray shield member 21, the base 7, the electrical circuit board 1, the element 3 and the protective layer 5, respectively, isX2=Sg·PsX3=Sp·UsXa=Su·As  (2)Xb=Su·BsXc=Su·Cs
Where Sg, Sp and Su are the amount of X-ray radiation passing through the glass plate 19, the X-ray shield member 21 and the base 7, respectively. Here, if the transmittivity at the glass plate 19 is Gt, then the amount of X-ray radiation Sg passing through the glass plate 19 isSg=S·Gt  (3)
Similarly, if the transmittivity of the X-ray shield member 21 and the base 7 is Pt and Ut, respectively, then the amount of X-ray radiation passing through the X-ray shield member 21 and the base 7, respectively Sp and Su, isSp=Sg·PtSg=Sp·Ut   (4)
Therefore, substituting the equations in (3) and (4) for each of the equations in (2) yieldsX2=S·Gt·PsX3=S·Gt·Pt·UsXa=S·Gt·Pt·Ut·As  (5)Xb=S·Gt·Pt·Ut·BsXc=S·Gt·Pt·Ut·Cs
The amount of scattered rays returning as far as the phosphor sheet 8 is shown as the sum of the amount of scattered rays from each layer. If the amount of scattered rays per unit of surface area returning from a position A on the electrical circuit board 1 is Ra, thenRa=X1+Gt·X2+Gt·Pt·X3+Gt·Pt·Ut·Xa  (6)Therefore,Ra=S·Gs+S·Gt2·Ps+S·Gt2·Pt2·Us+S·Gt2·Pt2·Ut2·As  (7)Similarly, if the amount of scattered rays per unit of surface area returning to the phosphor sheet 8 from a position B of the element 3 and a position C of the protective layer 5 is Rb and Rc, respectively, thenRb=S·Gs+S·Gt2·Ps+S·Gt2·Pt2·Us+S·Gt2·Pt2·Ut2·BsRc=S·Gs+S·Gt2·Ps+S·Gt2·Pt2·Us+S·Gt2·Pt2·Ut2·Cs  (8)In this manner, in order for scattered rays from constituent elements of the layer below the base 7 to return to the phosphor sheet 8, these scattered rays must pass through the base 7, the X-ray shield member 21 and the glass plate 19.
If the transmittivity Pt of the X-ray shield member 21 is low enough, then substantially all of the scattered rays are absorbed by the X-ray shield member 21 without reaching the phosphor sheet 8. As a result, the amount of scattered rays (that is, the amount of backscattering) from the lower layer can be held to insignificant levels at the X-ray image. Conversely, if the transmittivity of the X-ray shield member 21 is not low enough, then the differences in the amount of scattered rays from the electrical circuit board 1, the element 3 and the protective layer 5 (the effect of the last term in equations (7) and (8)) becomes exceptionally large and the differences between Ra, Rb and Rc appear as an image pattern in the X-ray image. Lead (Pb), with its X-ray low transmittivity, is commonly used as the material for the X-ray shield member 21. However, compared to other metals, lead lacks rigidity and is hard to handle, and for these reasons a member is required to support the lead, which complicates the structure of the apparatus. Moreover, the structure shown in FIG. 7 requires an X-ray shield member of approximately the same size as the two-dimensional array sensor, and therefore the use of high-density lead for the X-ray shield member 21 greatly increases the weight of the apparatus. In particular, with a portable (that is, hand-held) radiographic apparatus (also called an electronic cassette or a cassette-type radiographic apparatus) containing a two-dimensional array sensor, which is generally heavier than the conventional film cassette, the heavier the apparatus becomes the less portable it becomes and thus the harder it is to position correctly with respect to the object.